American Chemical Society
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Controlling the Cathodic Potential of KVPO4F through Oxygen Substitution

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posted on 2022-05-03, 20:13 authored by Romain Wernert, Long H. B. Nguyen, Emmanuel Petit, Paula Sanz Camacho, Antonella Iadecola, Alessandro Longo, François Fauth, Lorenzo Stievano, Laure Monconduit, Dany Carlier, Laurence Croguennec
Exploring and tailoring new high energy density positive electrode materials is still a challenge for alkali-ion batteries. In this work, we synthesized the mixed anion phases KVPO4F1–yOy (y = 0, 0.25, 0.5, 0.75, 1) and determined their crystallographic and electronic structures by combining synchrotron X-ray diffraction, X-ray absorption spectroscopy at the vanadium K edge, and 31P MAS NMR coupled with density functional theory calculations. These experiments confirmed that the substitution of F for O2– anions occurs as a solid solution across the whole composition domain. The local environments of vanadium ions are complex and diverse since the cis and trans octahedra undergo different distortions in the presence of a vanadyl bond. The simultaneous existence of ionic VIII–F bonds and covalent (VIVO)2+ vanadyl type entities is strongly affecting the electrochemical properties and potassium deinsertion/insertion mechanisms upon cycling. Ultimately, KVPO4F0.5O0.5 appears as a promising positive electrode material due to its high capacity (105 mAh·g–1), working potential (4.2 V vs K+/K), and sloping electrochemical curve.